Narrative:

After some en route delays/holding due to WX at sfo, we were being radar vectored for an ILS 19L approach at sfo. Our aircraft was assigned 6000' and heading 330 degrees. Flight conditions, in clouds with light to moderate rain. We were 20-25 mi northwest of sfo when both capts and first officer's radar altimeters (which detect ground clearance altitude between the rate of 0-2500') began indicating 2000'. Both radar altimeters continued to read and decreased to 800' on the captain's altimeter and 600' on the first officer's altimeter accompanied by an aural terrain warning from the GPWS. After several aural warnings and continued radar altitude decrease, a pull up maneuver was executed. We called ATC as soon as possible, but had reached 8000' before ATC acknowledged our radio calls. Actual aircraft altitude reached was about 8600' and then returned to 8000' as assigned by ATC. I am familiar with the area around sfo and had checked the MSA altitude prior to the problem. Even though I knew 6000' was a safe altitude, we were in an area of high terrain northwest of the bay in marin county. With definite terrain closure indications from the radar altimeters and GPWS, I felt I could not ignore them as false indications, especially with radar altimeter indications continuing to decrease below 800 and 600' AGL. So, a pull up maneuver was accomplished at the completion of the flight. The supervisor on duty for bay approach was called on the telephone. I explained my situation, the instrument indications and the reason I climbed from 6000 to 8000'. In retrospect, I feel the problem was caused by an erroneous input affecting both separate radar altimeters. The erroneous input to the RA's caused the GPWS to activate in mode 2 with a terrain warning. Callback conversation with reporter revealed the following: reporter said that he had learned nothing to explain the incident. He said that the rate of change of the radio altimeter was such that it could conceivably match an aircraft climbing below him. His quandary was that he had 3 altimeters all telling him that he had terrain clearance and a GPWS telling him that he should climb.

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Original NASA ASRS Text

Title: RADAR ALTIMETER SHOWED DECREASE IN ALT TO 600' FOLLOWED BY TERRAIN WARNING. FLT CREW CLIMBED ABOVE ASSIGNED ALT.

Narrative: AFTER SOME ENRTE DELAYS/HOLDING DUE TO WX AT SFO, WE WERE BEING RADAR VECTORED FOR AN ILS 19L APCH AT SFO. OUR ACFT WAS ASSIGNED 6000' AND HDG 330 DEGS. FLT CONDITIONS, IN CLOUDS WITH LIGHT TO MODERATE RAIN. WE WERE 20-25 MI NW OF SFO WHEN BOTH CAPTS AND F/O'S RADAR ALTIMETERS (WHICH DETECT GND CLRNC ALT BTWN THE RATE OF 0-2500') BEGAN INDICATING 2000'. BOTH RADAR ALTIMETERS CONTINUED TO READ AND DECREASED TO 800' ON THE CAPT'S ALTIMETER AND 600' ON THE F/O'S ALTIMETER ACCOMPANIED BY AN AURAL TERRAIN WARNING FROM THE GPWS. AFTER SEVERAL AURAL WARNINGS AND CONTINUED RADAR ALT DECREASE, A PULL UP MANEUVER WAS EXECUTED. WE CALLED ATC ASAP, BUT HAD REACHED 8000' BEFORE ATC ACKNOWLEDGED OUR RADIO CALLS. ACTUAL ACFT ALT REACHED WAS ABOUT 8600' AND THEN RETURNED TO 8000' AS ASSIGNED BY ATC. I AM FAMILIAR WITH THE AREA AROUND SFO AND HAD CHKED THE MSA ALT PRIOR TO THE PROB. EVEN THOUGH I KNEW 6000' WAS A SAFE ALT, WE WERE IN AN AREA OF HIGH TERRAIN NW OF THE BAY IN MARIN COUNTY. WITH DEFINITE TERRAIN CLOSURE INDICATIONS FROM THE RADAR ALTIMETERS AND GPWS, I FELT I COULD NOT IGNORE THEM AS FALSE INDICATIONS, ESPECIALLY WITH RADAR ALTIMETER INDICATIONS CONTINUING TO DECREASE BELOW 800 AND 600' AGL. SO, A PULL UP MANEUVER WAS ACCOMPLISHED AT THE COMPLETION OF THE FLT. THE SUPVR ON DUTY FOR BAY APCH WAS CALLED ON THE TELEPHONE. I EXPLAINED MY SITUATION, THE INSTRUMENT INDICATIONS AND THE REASON I CLBED FROM 6000 TO 8000'. IN RETROSPECT, I FEEL THE PROB WAS CAUSED BY AN ERRONEOUS INPUT AFFECTING BOTH SEPARATE RADAR ALTIMETERS. THE ERRONEOUS INPUT TO THE RA'S CAUSED THE GPWS TO ACTIVATE IN MODE 2 WITH A TERRAIN WARNING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR SAID THAT HE HAD LEARNED NOTHING TO EXPLAIN THE INCIDENT. HE SAID THAT THE RATE OF CHANGE OF THE RADIO ALTIMETER WAS SUCH THAT IT COULD CONCEIVABLY MATCH AN ACFT CLBING BELOW HIM. HIS QUANDARY WAS THAT HE HAD 3 ALTIMETERS ALL TELLING HIM THAT HE HAD TERRAIN CLRNC AND A GPWS TELLING HIM THAT HE SHOULD CLB.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.